Vertical ridge augmentation feasibility using unfixed collagen membranes and particulate bone substitutes: A 1‐ to 7‐year retrospective single‐cohort observational study

Abstract Aim To determine whether vertical ridge augmentation (VRA) can be obtained through guided bone regeneration (GBR) using exclusively resorbable collagen membranes and particulate bone substitutes without additional stabilization. Materials and Methods This study retrospectively examined 22 participants who underwent VRA with staged or simultaneous implant placement. The vertical defects of all participants were filled with particulate bone substitutes and covered with resorbable collagen membranes. The augmented sites were stabilized with unfixed collagen membranes and the flap without any additional fixation. The augmented tissue height was assessed using cone‐beam computed tomography at baseline, immediately after surgery, and at annual follow‐ups. Results The vertical bone gain of the 22 augmented sites amounted to 6.48 ± 2.19 mm (mean ± SD) immediately after surgery and 5.78 ± 1.72 mm at 1‐ to 7‐year follow‐up. Of the 22 augmented sites, 18 exhibited changes of less than 1 mm, while the other 4 showed changes of greater than 1 mm. Histological observation of three representative cases revealed new bone apposition on the remaining material. Conclusion The present findings indicate that GBR procedures using exclusively collagen membranes and particulate biomaterials without any additional fixation are feasible options for VRA.

• Vertical ridge augmentation could be obtained using simplified clinical procedures such as guided bone regeneration with resorbable membranes. However, clinical data supporting this hypothesis are currently scarce.

What this study adds
• The present study provides evidence on the feasibility of VRA using exclusively collagen membranes and particulate bone substitutes without any additional fixation.

| INTRODUCTION
Vertical ridge augmentation (VRA) is the most challenging intervention in implant dentistry, mostly due to its technical sensitivity and frequent complications. 1,2 Vertical ridge augmentation aims to regenerate bone volume at sites of the host chosen for implant placement, at which the bony walls are often missing. This is biologically challenging due to a lack of bony walls hindering blood clot stabilization 3 and access to osteoprogenitor cells, which may induce inadequate bone regeneration. One attempt to overcome this biological limitation involves using autogenous bone block grafts or distraction osteogenesis. 4,5 However, these procedures are surgically invasive and are associated with increased morbidity. 1 Therefore, simplifying the surgical procedures and reducing the invasiveness of VRA have become increasingly important.
This trend is further emphasized by the shift from specialists and referral-based clinicians to general dentists for implant dentistry. 6 In this sense, guided bone regeneration (GBR) seems to be a logical and wellknown alternative for these complex procedures.
Guided bone regeneration is a reliable and well-documented clinical procedure 7,8 that has been indicated as a viable alternative for VRA. 9 Although nonresorbable membranes are considered the standard reference for GBR because of their space-making capacity and controlled barrier function, 10 they are often associated with soft-tissue complications after exposure [11][12][13] and require additional surgery for their removal.
Consequently, resorbable membranes were proposed for VRA procedures, 9 despite their inherent lack of a space-making capability. To circumvent this lack of mechanical stability, resorbable membranes have been used alongside stabilizing devices (eg, pins, tenting screws, titanium mesh, or stabilizing sutures). However, these supportive devices can be difficult to install and must be removed in an additional surgery. Moreover, there is little evidence to support their use. A recent systematic review by the 15th European Workshop on Periodontology on Bone Regeneration found insufficient clinical evidence to identify the most effective technique for VRA. 14 This means that the gold standard for VRA is yet to be determined.
Even though no technique is superior to others regarding vertical augmentation, 14 focusing on and further exploring less-invasive techniques for VRA such as GBR is reasonable. It can be assumed that patients tend to prefer this strategy over more-complex procedures that cause complications and morbidity, leading to longer treatment times and higher costs. A recent relevant systematic review indicated that VRA is feasible regardless of which technique is used, 9 and it can therefore be hypothesized that VRA can also be obtained using simplified clinical procedures such as GBR with resorbable membranes. However, clinical data supporting this hypothesis are currently scarce.
Therefore, the aim of the present study was to determine whether VRA can be achieved by using a simplified GBR procedure with resorbable collagen membranes and particulate bone substitutes without additional stabilization.

| Study design and population
Participants were eligible for inclusion if they had received either staged or simultaneous vertical augmentation procedures in either the maxillary or the mandibular region. Only participants who underwent GBR using a resorbable collagen membrane without a fixation method or device were included. Subjects were excluded if they had received GBR using autogenous/allogenous block bone grafts, titanium mesh, nonresorbable membranes, or any type of fixation such as screws or bone tacks. Participants were all treated by the same experienced oral surgeon (Jung-Seok Lee) at the Department of Periodontology of Yonsei University Dental Hospital between 2014 and 2019. The study protocol was approved by the Institutional Review Board of Yonsei University Dental Hospital (approval no. 2-2021-0063), which abides by the Good Clinical Practice guidelines and the regulatory requirements. Due to the retrospective design of the study, informed consents were not necessary. The manuscript was prepared in accordance with the STROBE guidelines.

| Incisions
Midcrestal incisions were made on the keratinized gingiva that covered the entire vertically deficient ridge. Where gingival tissues had healed immaturely or unevenly, the incision line was displaced either buccally or palatally/lingually to include the defective soft tissue on one side of the flap to prevent it from perforating. One or two vertical-releasing incisions were made at least one tooth away from the surgical site, and a full-thickness mucoperiosteal flap was elevated beyond the margin of the bone defect to expose the entire defective area.

| Flap-stabilizing technique
The elevated buccal mucoperiosteal flap was advanced using periosteal-releasing incisions at the base of the flap for passive F I G U R E 1 Clinical photographs (A), panoramic radiographs (B), and cone-beam computed tomography (CBCT) imaging (C) illustrating vertical ridge augmentation (VRA) performed by guided bone regeneration (GBR) on the posterior maxilla. (A) A preoperative view indicating the vertically deficient ridges of the right posterior maxilla. A vertical bone defect was exposed after flap elevation. Sinus augmentation was performed using the lateral window technique. GBR was performed using particulate bone substitutes and resorbable collagen membranes. Membranes were placed over the bone substitute without using a fixation device. The flap was advanced using minimal releasing incisions to achieve tension-free primary closure and was sutured. Implant fixtures were installed. A harmonious appearance was observed in the

| Demographic information of the included cases
This study included 22 patients, of whom 11 were males and 11 females with an age of 50.09 ± 10.02 years (mean ± SD) (43-71 years; Table 1).
Of these patients, 14 were systemically healthy at the time of surgery,

| Histological observations
Histological biopsy samples were obtained using trephines during implant preparation for three sites of the 14 staged-approach cases that had been augmented using DPBM. Two samples were obtained from augmented single mandibular molar sites (cases 7 and 14).
These histological examinations indicated that substantial new bone formation occurred around the residual biomaterials ( Figure 5).  Although the present cohort presented a relatively high rate of complications (32%), these were not considered serious 14  The gain values obtained in the present study were lower than those found in other techniques used for VRA, such as distraction osteogenesis and bone blocks, which was consistent with previous reports. A systematic review with meta-analysis compared these two techniques with GBR and indicated that distraction osteogenesis produced the greatest bone gain (8.04 mm) but also the highest complication rate (47.3%). 14 In contrast, the same review revealed that GBR had the lowest complication rate (12.1%) and a substantial bone gain (4.18 mm). Although distraction osteogenesis shows the greatest bone gains, patients tend to prefer alternative treatments that have fewer complications, are less invasive, and avoid a donor-site surgery. 22 The reported bone gains using GBR are much smaller than those of distraction osteogenesis. Nevertheless, the clinician should not necessarily select the treatment option with greatest efficacy but rather the option with less morbidity in accordance with the patient's preferences. 23 Furthermore, due to the familiarity with the technique, GBR has become the most favored treatment choice by not only the referral-based clinicians but also the specialists. In this context, and based on the present findings, GBR utilizing the flap-stabilizing technique might be a viable method for VRA.
This study has some limitations that should be considered when interpreting the present findings. Firstly, it has a retrospective design, with a single operator and 1 cohort of 22 patients. Also, three different types of bone substitutes were used, which might interfere with the interpretations of the present results. It should be noted, however, that previous studies have shown comparable efficacy of these three bone substitutes. 19,24 Moreover, the mean difference (À0.7 ± 1.13 mm) between the different substitutes was minimal, indicating a similar regenerative capacity. Secondly, biopsy samples were not available for all cases, and bone regeneration could therefore not be confirmed despite the favorable radiographic results. Thirdly, most of the cases were still midway through their follow-up period. Finally, it is difficult to standardize the flap advancement method for the stabilization of the grafted materials. Therefore, more studies are needed with a prospective controlled trial design including multiple institutions and longer observation periods to further evaluate the exclusive use of collagen membranes and particulate bone substitutes as clinical modalities for VRA.
In the present 22 patient cohort, the exclusive use of collagen membrane and particulate bone substitutes resulted in ≈5 mm of VRA with a low complication rate. The histological bone formation and observed ridge stability in the present study indicates the feasibility of VRA by using exclusively collagen membranes and particulate bone substitutes.